1. Field of the Invention
The present invention relates to a water absorbing agent and an absorbent material. More specifically, it relates to a water absorbing agent with excellent absorption rate (initial absorbency) and planar diffusion of an absorbed liquid to provide an excellent surface dry feeling when used in an absorbent material, and an absorbent material using the water absorbing agent.
2. Description of the Prior Art
An absorbent material conventionally used comprises at least a liquid-permeable top sheet, a liquid-impermeable back sheet, and an absorbent layer containing a water absorbent resin and a fibrous material provided between the top and back sheets for absorbing and retaining a liquid. In order to improve the performance of absorbent materials using water absorbent resins, various proposals have been made, aimed at improving the feeling of surface dryness after absorbing a body fluid such as urine and blood, and reduction of leakage. Various ideas have been introduced in many aspects, including improvement of the surface non-woven fabric, arrangement of a water absorbent resin in the absorbent layer, ratio of a water absorbent resin and a fibrous material, application of a hydrophilic diffusion sheet such as a non-woven fabric or a water absorbing paper, and introduction of the embossing treatment of an absorbent layer and variation of the pattern of the embossing. However, these have not yielded a satisfactory result.
Particularly in recent years, since a higher ratio of a water absorbent resin to a fibrous material is provided in an absorbent layer and the weight of the absorbent layer is reduced (the amount of the fibrous material is reduced and the amount of the water absorbent resin is increased) to make the absorbent material thinner and light weight, the efficient use of whole area of absorbent layer is an important element.
In particular, improvement in the absorption rate and the planar diffusion of a liquid to be absorbed is the key for the improvement in the feeling of surface dryness after absorption, and the reduction of leakage.
Efforts have been made to improve diffusion of a liquid by improving a water absorbent resin in an absorbent layer, such as:
1 modifying a water absorbent resin or an absorbent layer with a silicone oil modified by ethylene oxide to apply hydrophilic characteristics as disclosed in JP-8-289903-A, PA1 2 using a water absorbing agent comprising a water absorbent resin with a comparatively coarse particle size of 400 to 1680 .mu.m in the mass median particle size as disclosed in JP-A-1-132802, and PA1 3 improving the diffusion of a liquid by increasing the gel strength and preventing the gel blocking by the use of a mixture of silicon dioxide and a water absorbent resin crosslinked by a first crosslinking agent having at least two double bonds copolymerizable with acrylic acid and a second crosslinking agent having at least two functional groups reactive with a carboxylic acid group as disclosed in JP-A-7-88171.
However, in the case of the above-mentioned item 1, the need of an extra process of treating with the modified silicone oil causes problems of complicating the process and cost increase.
In the case of the item 2, although it is effective to some extent in terms of the diffusion of a liquid, the absorption rate of the water absorbent resin itself is deteriorated due to the large particle size, and thus it involves a problem of insufficient dry feeling after the diffusion of the liquid.
In the case of the item 3, since the crosslinking degree needs to be increased in general, for increasing the gel strength, the absorbency of the water absorbent resin decreases so that a large amount of the water absorbing agent is required for obtaining the absorbing capacity necessary for an absorbent material, and thus it is not economical.